Could anyone please provide me some guidance in how to plan a sample for a Welch's t test (with some references)? One can find a plenty of information online for a Student's t test. But I couldn't find anything about the calculation for a Welch's t test.
I've heard the power of a Welch's test is similar to the power of a Student's one. But I also wasn't able to find how to calculate the power (so I could derive the calculation of the sample size). And also, I couldn't find confirmation that they are similar enough for me to use the same procedures as in the Student's test.
I appreciate any answers!!
My idea is to use the Minitab software to estimate sample size. I've posted a video for you.
Hello Bruno. You haven't indicated what software you use. If you happen to have Stata, take a look at -power twomeans-. Example 2 shows how to use the sd1() sd2() options. Note that it estimates the sample size for Satterthwaite's t-test, not Welch's t-test; but the two usually give pretty similar results in my experience. There is also an nratio option to specify the n2/n1 ratio (the default being 1/1 if the option is excluded). See Example 5. HTH.
- https://www.stata.com/manuals13/psspowertwomeans.pdf
Bruno
If you are planning to use equal size samples, use formulas for Student’s T-test. It is the most powerful one in this case and when samples sizes are equal it is very robust against moderate variances difference.
If you have some idea about variances difference, you can use GPower, a free and very good program for sample size calculation. You can easily find it in the internet.
Jorge Ortiz
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